1. Field of the Invention
This invention relates generally to semiconductor optical amplifiers. More particularly, it relates to vertically lasing semiconductor optical amplifiers (VLSOAs) used in combination with a photodetector to create an improved optical receiver.
2. Description of the Related Art
Fiber optic systems are used to transmit information at high speeds over large distances. At a high level, a typical optical system consists of an optical transmitter and an optical receiver with an optical fiber connecting the two. The optical transmitter converts an electrical signal into an optical signal and transmits the signal over the optical fiber. The optical receiver receives the optical signal from the optical fiber and converts the signal back into an electrical signal.
For current systems running at bit rates less than 10 billion bits per second (Gbps), a common design for the optical receiver is based on a PIN diode coupled to a transimpedance amplifier (TIA). The optical signal enters the PIN diode and creates electron-hole pairs inside the PIN diode. The electrons and holes accelerate to opposite ends of the PIN diode resulting in a flow of current that varies with the strength of the incoming optical signal. The electrical signal resulting from this flow is then output to the TIA, where the signal is amplified and output to further electronic circuitry.
However, as bit rates increase above 10 Gbps, optical amplifiers are desirable to pre-amplify the optical signal before it reaches the photodetector. In other words, it is difficult to produce a high sensitivity, high gain electronic amplifier capable of operating at these higher data rates. As a result, it becomes more desirable to provide amplification optically via an optical amplifier located before the detector rather than electronically via an electronic amplifier located after the detector. This optical pre-amplification results in improved receiver sensitivity and a larger amplitude photocurrent at the photodetector. This larger photocurrent can then be amplified further, or in some cases directly input into further electronic circuitry. Pre-amplification also increases the signal to noise performance of the overall system in certain types of detection systems. Optical amplifiers are also used to adjust the power of the incoming signal to match the operating region of the photodetector. For example, a typical power range is −3 to −14 dBm for common PIN diodes, —3 to −18 dBm for more expensive PIN diodes, and —9 to −24 dBm for avalanche photodiodes (APDs).
Fiber amplifiers are one type of optical amplifier. Fiber amplifiers include a length of fiber which has been doped to form an active gain medium. Ions of rare-earth metals, such as erbium, are typically used as the dopant. The doped fiber is typically pumped by an optical pump at a wavelength which is preferentially absorbed by the ions but different from the wavelength of the optical signal to be amplified. The pumping results in a population inversion of electronic carriers in the active medium. Then, as the optical signal propagates through the doped fiber, it is amplified due to stimulated emission.
One drawback of fiber amplifiers is that they typically only operate over a narrow wavelength range when multiple fiber amplifiers are cascaded. This is especially problematic if the optical signal to be amplified covers a wide range of wavelengths, as would be the case if the entire bandwidth of the optical fiber is to be efficiently utilized. Another disadvantage of fiber amplifiers is their transient response to channel drop-out in wavelength division multiplexing systems. Further problems with fiber amplifiers include slow switching speed, power inefficiency, difficulties in mass producing them, and their high cost which makes them prohibitively expensive for many applications. Another major drawback of fiber amplifiers is their inherently large size.
Semiconductor optical amplifiers (SOAs) are another type of optical amplifier. SOAs contain a semiconductor active region and an electrical current typically is used to pump the electronic population in the active region. An optical signal propagating through the active region experiences gain due to stimulated emission.
Conventional SOAs are non-lasing. One problem with non-lasing semiconductor optical amplifiers is that the gain depends on the amplitude of the optical signal. This problem is the result of gain saturation, in which there are insufficient carriers in the conduction band to provide the full amount of gain to higher power signals. As a result, a strong optical signal is amplified less than a weak signal and strong portions of the optical signal are amplified less than weak portions. This results in distortion of the optical signal and the possibility of crosstalk between different optical signals propagating simultaneously through the system. This significantly limits the use of conventional SOAs, especially in optical systems operating at high speeds and in wavelength division multiplexed optical systems. Further, the non-linearity of the amplification in conventional SOAs leads to crosstalk between bits in high frequency time division multiplexed (TDM) systems (commonly referred to as intersymbol interference). Due to the TDM crosstalk, non-lasing SOAs are typically limited to output powers below 1 mW or bit rates well below 2.5 Gbps.
What is needed is an optical receiver, including an optical amplifier, that is small in size, inexpensive and can be used to detect time division multiplexed (TDM) and wavelength division multiplexed (WDM) optical signals at high bit rates, for example, bit rates greater than 10 Gbps.